This invention relates generally to sterling silver alloy compositions of increased hardness, and more particularly is directed to sterling silver manganese alloy compositions which exhibit an exceptional and reversible hardness.
The background information discussed below is presented to better illustrate the novelty and usefulness of the present invention. This background information is not admitted prior art.
Silver metal is very ductile and malleable (being only slightly harder than gold) and is the most lustrous metal on Earth. Silver's brilliant white metallic luster can take a high degree of polish making silver highly desirable in the production of jewelry and tableware. Silver's unique properties are important in the decorative arts, coinage, industry, and photography, for example.
Silver categorized as “fine silver” comprises at least 99.5 percent pure silver. Fine silver is relatively soft having a Moh's hardness of about 2.5, a Brinell hardness of about 24.5 MN m−2, and a Vickers hardness of about 251 MN m−2. Thus, fine silver is generally too soft for the production of large, functional objects. Although the malleability of fine silver permits it to be easily shaped into attractive forms, products made with pure soft silver are easily dented or bent out of shape.
Fine silver has good tarnish resistance. In fact, the tarnish resistance of silver alloys increases as the percentage of fine silver increases, as pure silver is unreactive in clean air under normal conditions and unreactive with clean water. Because of it softness and malleability, however, fine silver is commonly combined with other metals to produce more durable products, thus increasing its susceptibility to tarnish.
When fine silver is combined with other metals to form a new material, the new material is referred to as an alloy. Alloyed silver cannot be classified as sterling silver unless it consists of at least 92.5 percent fine silver, whereas the remaining 7.5 percent may be a combination of other elements in various proportions. Sterling silver alloy is the material of choice where appearance is paramount and strength is important, such as in the manufacture of jewelry, coinage, and silverware. Sterling silver sets the standard for high quality silver products. In addition to offering increased strength and durability, products made with sterling silver will not wear away, as silver plating can.
The most common sterling silver alloy consists of at least 92.5 percent silver and up to 7.5 percent copper. Adding copper to silver, to produce a copper sterling silver alloy improving hardness and durability while maintaining the beautiful color of the pure silver, is a well-known practice in the art of silver manufacture. The difference between the softness of fine silver and the hardness of copper-sterling silver alloy is such that the practice is widely, if not nearly ubiquitously, used in the vast majority of sterling silver production because of the great need in the industry for material that is harder than fine silver. Despite the improvement in hardness when copper is added to fine silver to produce a copper silver alloy, there still exists a need in the industry for a harder as-cast sterling silver, particularly in the jewelry part of the industry as most of jewelry, such as earrings, rings, pendants, and the like are formed as an as-cast product. Improvement in the as-cast hardness would result in the jewelry being less susceptible to damage, such as bending, in addition to exhibiting better wear resistance and polishing ability.
While the small amount of copper that is added to fine silver produces an alloy with increased hardness and durability, the presence of copper in the alloy introduces a susceptibility to tarnishing. Copper tarnishes much more readily than does silver as copper, unlike silver, has a great affinity for oxygen. When copper reacts with oxygen, it typically forms a copper oxide that may consist of cupric or cuprous oxide, or both. When copper oxide forms on the copper that is alloyed with sterling silver, it is known as “fire scale.” Fire scale is typically a darkened portion of the sterling silver piece that may result from melting or brazing, and, in fact, each time the alloy is heated, such as when the alloy is initially formed as shot, when the shot is melted and recast to form the desired article, and subsequently when the cast article is annealed. Fire scale, as compared to silver tarnish, is not limited to the surface of the sterling silver object, but may penetrate the article to some depth and, thus, may not be removable by buffing and polishing. Additionally, unless air is excluded during the casting process, the cast article may contain internal voids which, of course, can lead to undesirable porosities and grain sizes in the cast article.
There have been some attempts at alleviating some of the aforementioned problems associated with conventional sterling silver alloys. Early attempts to provide silver alloys having tarnish resisting properties and improved workability were concerned with either silver alloys that did not contain enough silver to qualify as sterling silver or with fine silver alloys, i.e., nearly pure silver. Silver alloys containing less than 92.5 percent pure silver are not of interest to the jewelry making industry because this silver cannot be labeled as sterling. Conversely, fine silver is not of interest to the jewelry making industry because it is too soft and too expensive.
Attempts at providing for sterling silver alloys and master alloys having certain desired properties, such as reduced fire scale production, reduced porosity, and reduced grain size over conventional sterling silver alloys have met with some success. Not surprisingly, these sterling silver alloys have been widely used by the industry especially for the production of silver jewelry. Despite these improvements, however, sterling silver alloys are still relatively soft and cannot provide the desired degree of as-cast hardness and the desired reversible heat treatability performance that is required by many industries that rely on the use of silver, such as the jewelry making industry.
Thus, it is appreciated that there is still an unmet need for a sterling silver alloy that possesses an increased as-cast hardness, in addition to being reversibly heat treatable, yet maintaining a substantially reduced formation of fire scale upon heating, a decrease in voids and porosity, and a reduced grain size relative to traditional sterling silver alloys.